The use of an automated growth analyser to measure recovery times of single heat-injured Salmonella cells

A new approach to the study of recovery times of single heat-injured Salmonella cells is described. It comprises the generation of a standard heat-injured culture, serial dilution of this culture to near extinction, inoculation of the serial dilutions across many microtitre plates and measurement of the subsequent recovery and growth using an automated turbidometric analyser. Lag times for individual cells were estimated from turbidity data using a model that accurately extrapolated the growth curve back to the starting inoculum level. Lag times were compared using a number of different commercially available pre-enrichment media. The most typical result was a very broad distribution of lag times at the single cell inoculum level, with many values in excess of 20 h. Even at an inoculum level 10-fold higher, lag times for some injured cells were estimated to be > 10 h. More significantly, it was found that some media recovered more injured cells than others and vice versa. Between the worst and best media there were as many as 3 log(10) cycles difference in the number of cells recoverable. No trends were apparent linking choice of medium with performance. The implications of these findings, in relation to traditional and rapid methodology, are discussed.